Method and device for treating sludge

ABSTRACT

A method for treating sludge by dewatering to form thick sludge and drying to form dry sludge includes the steps of mechanically predewatering thick sludge to produce predewatered thick sludge and mixing the predewatered thick sludge with non-cooled dry sludge to produce a sludge mixture. The temperature of the sludge mixture to be dried is measured and the dry substance contents of the dry sludge based on the measured temperature is determined. Fluctuations in the dry substance contents of the dry sludge are compensated by controlling the amount of recirculated dry sludge to be mixed with the predewatered thick sludge as a function of the temperature of the sludge mixture to be dried. The sludge mixture is dried to produce dry sludge and a portion of the dry sludge is recirculated to be mixed with the predewatered thick sludge.

BACKGROUND OF THE INVENTION

The invention relates to a method and a device for treating sludge,especially sewage sludge, whereby dry sludge is mixed into mechanicallypredewatered thick sludge, the mixture is dried to form dry sludge, andthe dry sludge is cooled before being transported away.

Such a sludge drying serves to condition the sludge primarilyoriginating in water (sewage) treatment plants such that later on,optionally after intermediate storage, it can be disposed of, forexample, by combustion in a refuse incinerator or as fertilizer inagriculture.

The sewage sludge, in general, contains an amount of dry substance ofapproximately 2 to 5 mass %. The mechanical predewatering processincreases the dry substance contents to approximately 20 to 30 mass %.This thick sludge cannot be subjected directly to the actual dryingprocess. Indeed, the drying process requires a dry substance contents ofapproximately 50 to 80 mass %. Therefore, the thick sludge is mixed withalready dried sludge. During drying the dry substance contents of themixture is increased to approximately 95 mass %. During this process thetemperature of the dry sludge increases to approximately 120° C. Ingeneral, this temperature must be lowered before transporting the drysludge to approximately 50° to 70° C. because otherwise an unacceptablyhigh risk of self ignition is present.

European patent application 0 458 221 describes a method and a device ofthe aforementioned kind. Here, the temperature of the dryer iscontrolled by adjusting the mixture ratios between the thick sludge andthe dry sludge. For this purpose, the gas temperatures of therecirculating gas is determined and fed into the control device. Thelatter controls the metering device and thus the amount of dry sludge tobe admixed.

It has been found that the conventional sludge treatment needsimprovement with respect to economic efficiency.

Accordingly, it is an object of the invention to provide a possibilityfor sludge treatment with increased efficiency.

SUMMARY OF THE INVENTION

The method for treating sludge by predewatering to form thick sludge anddrying to form dry sludge according to the present invention isprimarily characterized by:

a) mechanically predewatering thick sludge to produce predewatered thicksludge;

b) mixing the predewatered thick sludge with non-cooled dry sludge toproduce a sludge mixture;

c) measuring the temperature of the sludge mixture to be dried;

d) determining the dry substance contents of the dry sludge based on themeasured temperature;

e) compensating fluctuations in the dry substance contents of the drysludge by controlling the amount of recirculated dry sludge to be mixedwith the predewatered thick sludge as a function of the temperature ofthe sludge mixture to be dried;

f) drying the sludge mixture to produce dry sludge; and

g) recirculating a portion of the dry sludge to step b).

The method further includes the step of adding dust, produced during thesludge treating process, to the predewatered thick sludge or the sludgemixture.

The step of drying preferably includes the step of employing a fluidizedbed dryer and wherein, after drying, the dry sludge contains dust of aparticle size of less than 500 μm in an amount of less than 20 mass %for use in the step of adding dust.

Advantageously, the method further includes the step of removing dustfrom the circulating dryer medium of the fluidized bed dryer for use inthe step of adding.

Expediently, the method further comprises the step of recirculating,after removing dust therefrom, a portion of the exhaust vapor, producedduring drying, into the fluidized bed dryer as a circulating dryermedium for use in the step of adding dust.

The method may also comprise the step of cooling the remaining portionof dry sludge, wherein the step of cooling includes the steps ofemploying a fluidized bed cooler and removing dust, for use in the stepof adding, from the circulating cooler medium of the fluidized bedcooler.

In another embodiment of the inventive method, the aspiration air,generated by the mechanically predewatered thick sludge, is used as thecirculating cooler medium.

The step of drying may include the step of employing a drum dryer and,after drying, the dry sludge has preferably a dust contents of aparticle size of less than 500 μm of less than 20 mass %.

The method may further include the step of removing the dust from thecirculating dryer medium of the drum dryer for use in the step ofadding.

The method advantageously further comprises the step of recirculating,after removing dust particles, a portion of the exhaust vapor producedduring drying into the drum dryer as a circulating dryer medium.

The present invention also relates to a device for performing theafore-described method. The device of the present invention is primarilycharacterized by:

at least one mixer;

a first inlet line connected to the mixer for supplying mechanicallypredewatered thick sludge;

a second inlet line connected to the mixer for supplying dry sludge;

a dryer connected downstream of the mixer;

a means for measuring the temperature of the sludge mixture;

a metering device for adjusting the throughput of dry sludge through thesecond inlet line;

a control device, connected to the means for measuring the temperatureand to the metering device, for controlling as a function of thetemperature the metering device for adjusting the throughput of the drysludge.

In a preferred embodiment the device further comprises a deductingdevice connected to the dryer and the mixer, wherein the dryer isselected from the group of a fluidized bed dryer and a drum dryer andwherein exhaust vapor of the dryer is guided through the deductingdevice and partially recirculated into the dryer.

In another embodiment the device further comprises a fluidized bedcooler connected downstream to the dryer, the cooler comprising an airinlet line for connecting the cooler to a thick sludge reservoir and anair outlet line, and a deducting device connected to the mixer, whereinthe air outlet line of the cooler is connected to the deducting device.

The dry substance contents of the thick sludge is not constant but issubject to fluctuations. These fluctuations however could not beautomatically compensated in the past. The invention is based on therecognition that upon admixing hot dry sludge to the thick sludge thedry substance contents affects the temperature of the mixture. Bymeasuring the temperature of the mixture it is thus possible to detectpossible fluctuations of the dry substance contents of the thick sludgeand to compensate them. This results in constant operating conditions ofthe actual drying step, so that the drying step can be maintained at anoptimal efficiency. The measures required for this, i.e., the measuringof the temperature of the mixture and the corresponding control of thedosage of the dry sludge, are very simple. For a recirculation of cooleddry sludge the temperature of the mixture does not change substantiallyso that the temperature cannot perform the function of the leadparameter.

Preferably, dust, especially fine dust, is admixed to the thick sludge,or, the mixture to be dried, which dust is produced during the sludgetreatment process, especially during the drying step and upon furthertreatment of the dry sludge. Repowdering thus is carried out by using adry substance which otherwise would require a special further treatment.

Advantageously, the mixture is dried in a fluidized bed dryer or a drumdryer such that the dry sludge directly after drying has a fine dustcontents of less than 20 mass % of a particle size of less than 500 μmin order to improve explosion protection.

In this context it is especially advantageous that the dust is removedfrom the circulating drying medium of the fluidized bed or the drum andis admixed to the thick sludge or the mixture to be dried. Thecirculating dryer medium is preferably a portion of the exhaust vapor,resulting from the drying process, which portion is recirculated.

It is furthermore especially advantageous that, upon performing coolingin a fluidized bed downstream of the dryer, dust is removed from thecirculating fluidized bed dryer medium so that a further reduction ofthe fine dust contents can be achieved. In general, the thick sludge isstored intermediately whereby the aspiration air in the form of cold airis removed from the thick sludge reservoir. Thus, the circulatingfluidized bed dryer medium for the cooling step is preferably in theform of this aspiration air which is produced by the mechanicallypredewatered thick sludge, as long as the methane contents of theaspiration air is not too great.

Of course, for the drying and cooling steps it is possible to use othermethods than the fluidized bed or drum method; however, other methodsare not quite as favorable.

The inventive sludge treatment device comprises at least one mixer whichhas an inlet line for mechanically prewatered thick sludge and an inletline for dry sludge, further a dryer positioned downstream of the mixer,and a cooler positioned downstream of the dryer. This device ischaracterized in that the inlet line for the dry sludge leading into themixer branches off upstream of the cooler, i.e., is branched off aconnecting line connecting the dryer with the cooler or is in the formof a separate line extending from the dryer. Furthermore, a controldevice is provided which is connected with a temperature measuring meansat the mixer as well as with a metering device which controls thethroughput of dry sludge within the line leading into the mixer. Themixer is thus loaded with non-cooled dry sludge so that the dryer can bedesigned to have a correspondingly reduced output. Furthermore, thecooler must only cool the amount of dry sludge that is to be removed asfinished product. Accordingly, the cooler can also be dimensioned to acorrespondingly reduced output. With the inventive mixing control, it ispossible with simple means to compensate for changes in the drysubstance contents of the thick sludge with the corresponding admixtureof dry sludge and to thereby maintain an optimal efficiency for thedrying step.

In a further embodiment of the invention it is suggested that the dryeris a fluidized bed dryer the exhaust vapor of which is guided through adedusting device and partially recirculated and that the dedustingdevice is connected to the mixer. The dust which is collected upondrying is thus used for the repowdering step so that a separate furtherdust treatment is obsolete.

Preferably, when using a cooler downstream of the dryer, the cooler isin the form of a fluidized bed cooler and connected via an air inletline to a thick sludge reservoir as well as via an air outlet line to adedusting device whereby the dedusting device is connected to the mixer.Thus, the dust which is produced during cooling is also used forrepowdering whereby the aspiration air removed from the thick sludgereservoir is used as the circulating fluidized bed cooler medium. Thisresults in a corresponding reduction of the amount of air to be disposedof.

Other combinations of the inventive features, which deviate from thepreviously discussed combinations, are also considered important to thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention will be described with the aid of apreferred embodiment in connection with the attached drawings. Thedrawings show a schematical representation of the inventive device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The device comprises as a main component a fluidized bed dryer 1 and afluidized bed cooler 2. Upstream of the fluidized bed dryer 1 a mixer 3is arranged which is supplied via an inlet line 4 with thick sludge, ingeneral via mass-flow controlled pumps for thick materials. This thicksludge is mixed with non-cooled dry sludge whereby the latter isintroduced into the mixer 3 via line 5 which branches off between thefluidized bed dryer 1 and the fluidized bed cooler 2.

The thick sludge is introduced into the mixer 3 with a dry substancecontents of 25 mass %. The hot dry sludge which is introduced into themixer 3 has a dry substance contents of 95 mass %. The supply of drysludge is adjusted such that the fluidized bed dryer 1 contains amixture with a dry substance contents of preferably 75 mass %. Withrespect to the dry substance to be introduced into the fluidized bedcooler 2, a ten-fold amount is recirculated through the line 5. Comparedto a device in which the recirculating amount is branched off downstreamof the cooler, the fluidized bed cooler 2 can, according to the presentinvention, be dimensioned so as to have an approximately 90% loweroutput. Furthermore, the heating output of the fluidized bed dryer 1 canbe reduced since the recirculating dry sludge is already at the dryertemperature, i.e., is already heated to approximately 120° C.

The inventively controlled recirculation of the hot dry sludgefurthermore has the special advantage that the drying process can becontrolled with simplest means. For this purpose a control device 6connected to a temperature measuring means within the mixer 3 and ametering device 7 in the line 5 is provided. By determining thetemperature within the mixer 3 and correspondingly controlling themetering device 7, the dry substance contents of the mixture introducedinto the fluidized bed dryer 1 can be maintained constant so that theunavoidable fluctuations of the dry substance contents of the thicksludge can be compensated. The temperature fluctuation range isapproximately 5° to 8° C.

The exhaust vapor produced within the fluidized bed dryer 1 isintroduced into a dedusting device 8. A portion of the exhaust vaporthat has been dedusted is recirculated as a fluidized bed dryer mediuminto the fluidized bed dryer 1. The remaining exhaust vapor is disposedof. The dust which is collected within the dedusting device 8 istransported into the mixer 3 and accordingly included into therepowdering step.

Aspiration air is used as a fluidized bed cooler medium for thefluidized bed cooler 2 which is sucked from a non-represented thicksludge reservoir, however, only as long as the methane contents of theaspiration air does not surpass a safety limit value. Otherwise, thedevice is switched to sucking in surrounding air. The air exiting fromthe fluidized bed cooler 2 and containing dust is introduced into adedusting device 9 and is subsequently disposed of. The dust from thededusting device 9 is also introduced into the mixer 3 and is thuscontained within the sludge granules to be introduced into the fluidizedbed dryer 1.

The dedusting of the dry sludge is especially important with respect toexplosion prevention. The inventive recirculation of the dust makesobsolete an otherwise required fine dust storage, optionally with anupstream filtration step. The mixer 3 can serve to receive all otherdusts that are collected within the further treatment course.

Other embodiments can be envisioned within the scope of the invention.Even though fluidized bed devices are especially advantageous for dryingand cooling, it is possible to envision other designs. Furthermore, itis possible to employ different fluidized bed dryer and cooler media.The essential advantages of the process design and control areindependent of the dust recirculation even though the latter provides aconsiderable contribution to explosion prevention and economicefficiency.

The present invention is, of course, in no way restricted to thespecific disclosure of the specification and drawings, but alsoencompasses any modifications within the scope of the appended claims.

We claim:
 1. A method for treating sludge by predewatering to form thicksludge and drying to form dry sludge, said method comprising the stepsof:a) mechanically predewatering thick sludge to produce predewateredthick sludge; b) mixing the predewatered thick sludge with non-cooleddry sludge to produce a sludge mixture; c) measuring the temperature ofthe sludge mixture to be dried; d) determining the dry substancecontents of the dry sludge based on the measured temperature; e)compensating fluctuations in the dry substance contents of the drysludge by controlling the amount of recirculated dry sludge to be mixedwith the predewatered thick sludge as a function of the temperature ofthe sludge mixture to be dried; f) drying the sludge mixture to producedry sludge; and g) recirculating a portion of the dry sludge to step b).2. A method according to claim 1, further including the step of addingdust, produced during the sludge treating process, to one of thepredewatered thick sludge and the sludge mixture.
 3. A method accordingto claim 2, wherein the step of drying includes the step of employing afluidized bed dryer and wherein, after drying, the dry sludge containsdust of a particle size of less than 500 μm in an amount of less than 20mass % for use in said step of adding dust.
 4. A method according toclaim 2, further including the step of removing dust from thecirculating dryer medium of the fluidized bed dryer for use in said stepof adding.
 5. A method according to claim 2, further comprising the stepof recirculating, after removing dust, a portion of the exhaust vapor,produced during drying, into the fluidized bed dryer as a circulatingdryer medium for use in said step of adding dust.
 6. A method accordingto claim 2, further comprising the step of cooling the remaining portionof dry sludge, wherein said step of cooling includes the steps ofemploying a fluidized bed cooler and removing dust, for use in said stepof adding, from the circulating cooler medium of the fluidized bedcooler.
 7. A method according to claim 6, further comprising the step ofusing the aspiration air, generated by the mechanically predewateredthick sludge, as the circulating cooler medium.
 8. A method according toclaim 2, wherein the step of drying includes the step of employing adrum dryer and wherein, after drying, the dry sludge has a dust contentsof a particle size of less than 500 μm of less than 20 mass %.
 9. Amethod according to claim 8, further including the step of removing thedust from the circulating dryer medium of the drum dryer for use in saidstep of adding.
 10. A method according to claim 8, further comprisingthe step of recirculating, after removing dust particles, a portion ofthe exhaust vapor produced during drying into the drum dryer as acirculating dryer medium.
 11. A device for treating sludge, said devicecomprising:at least one mixer; a first inlet line connected to saidmixer for supplying mechanically predewatered thick sludge; a secondinlet line connected to said mixer for supplying dry sludge; an outletline for transporting a resultant sludge mixture from said mixer to adryer connected downstream of said mixer; a means for measuring thetemperature of the sludge mixture; a metering device for adjusting thethroughput of dry sludge through said second inlet line; a controldevice, said control device being connected to said means for measuringthe temperature and to said metering device, wherein said control deviceadjusts the throughput of dry sludge through said metering device as afunction of the temperature of the sludge mixture.
 12. A deviceaccording to claim 11, further comprising a deducting device connectedto said dryer and said mixer, wherein said dryer is selected from thegroup consisting of a fluidized bed dryer and a drum dryer, whereinexhaust vapor of said dryer is guided through said deducting device andpartially recirculated into said dryer.
 13. A device according to claim11, further comprising:a fluidized bed cooler connected downstream ofsaid dryer, said cooler comprising an air inlet line for connecting saidcooler to a thick sludge reservoir and an air outlet line; and adeducting device connected to said mixer, wherein said air outlet lineof said cooler is connected to said deducting device.